Substations in high and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switches, power transformers and instrument transformers, which can be arranged in switch yards and/or bays. These primary devices can be operated in an automated way via a Substation Automation (SA) system. The SA system includes microprocessor based, programmable secondary devices, so-called Intelligent Electronic Devices (IEDs) responsible for protection, control and monitoring of the primary devices. The IEDs can be assigned to one of three hierarchical levels, e.g., the station level, the bay or application level, and the process level being separated from the bay level by a process interface. The station level of the SA system includes an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and a gateway to a Network Control Center (NCC). IEDs on the bay level, also termed bay units, in turn can be connected to each other and to the IEDs on the station level via an optical inter-bay or station bus.
IEDs on the process-level include for example can include application-specific (e.g., non-conventional) electronic sensors for voltage, current and gas density measurements, contact probes for sensing switch and transformer tap changer positions, or intelligent actuators for controlling switchgear like circuit breakers or disconnectors. Breaker-IEDs, if shielded against electromagnetic disturbances, can even be directly integrated into the switchgear or respective intelligent primary equipment. Such process-level IEDs can be connected to the bay units via a process bus, preferably optical, which can be considered as the process interface replacing the hard-wired process-interface that is known to connect the switchyard to the bay level equipment.
A communication standard for communication between the secondary devices of a substation has been introduced by the International Electrotechnical Committee (IEC) as part of the standard IEC 61850 entitled “communication networks and systems in substations”. For non-time critical messages, the communication between the IEDs is handled via a Manufacturing Message Specification (MMS) application level protocol and a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and the Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet as physical media. On the other hand, time critical messages, so-called Generic Object Oriented Substation Events (GOOSE, part IEC 61850-8-1 of the standard) build directly on the Ethernet link layer of the communication stack. Very time-critical signals at the process level such as trip commands and measured analogue voltages or currents use a simplified variant of GOOSE known as Sampled Values (SV, part IEC 61850-9-2 of the standard) that also builds directly on the Ethernet link layer.
Thus, switched Ethernet systems can be used for process control systems, e.g., to interconnect the secondary devices, to avoid message collisions as well as to restrict the data flow of multicast messages from and to secondary devices with virtual local area networks (VLAN) and multicast address based filters. Ethernet multicast messages in switched Ethernet can be in principle sent to all output ports of a network interconnection device such as a switch and to all receivers, e.g., secondary devices, in a LAN or VLAN, if no special filters can be set up on a network interconnection devices or a secondary device. These filters can be based on VLAN configuration and multicast address filters, which can be statically configured in the switches, for example.
The IEC 61850 multicast messages can be periodically sent, and sampled value messages even with sampling rates of 4 kHz and above. This leads to a high load not only in the Ethernet network, but on the application devices, e.g., the secondary devices, as every received packet ties up capacities of the secondary device, for example processing time at a network stack connecting the secondary device to the Ethernet and at a processor or central processing unit and the according application running on the secondary device. A load restriction at the Ethernet and/or the application IED can be gained by the static VLAN and/or multicast filter configuration in the switch or network interconnection device to which the IED is connected, or by a static filter on the IED. A static filter can be setup during the IED startup.